The present invention relates to an ultrasonic pulse Doppler blood flow meter which is associated with an operation checking mechanism.
An ultrasonic pulse Doppler blood flow meter serves to transmit by an ultrasonic transducer an ultrasonic wave into the body, receive its echoes, extract only echoes from a blood flow corpuscle or cell at the position to be measured of the received echoes, obtain a Doppler frequency shift from the extracted echoes, and perform a spectrum analysis on the echoes, thereby obtaining the blood flow velocity. More specifically, the Doppler frequency shift fd can be expressed by the following equation: EQU fd=[(2V.multidot.cos .theta.)/C].multidot.fc (1)
where
V: the flow velocity of corpuscle (i.e., blood flow velocity), PA1 .theta.: the angle between the direction of the ultrasonic beam and the direction of the blood flow, PA1 C: velocity of sound in tissue PA1 fc: the central frequency of the ultrasonic wave transmitted.
From the equation (1), it is understood that the flow velocity V of the blood is proportional to the Doppler frequency shift fd. The Doppler blood flow meter obtains the blood flow velocity V in view of this relation by obtaining the Doppler frequency shift of the echoes of the blood corpuscle.
An example of a conventional such flow meter is shown in FIG. 1. FIGS. 2A to 2D are time charts of the waveforms of the signals in the respective sections of the flow meter shown in FIG. 1. A clock pulse a (FIG. 2A) of a predetermined frequency is produced from a clock pulse generator 1. A rate pulse generator 2 receives the clock pulse a from the clock pulse generator 1 and produces a rate pulse b (FIG. 2B) of the period corresponding to the period of the ultrasonic wave (the driven period of an ultrasonic transducer 4). The rate pulse b is applied to a pulser 3 and a range gate circuit 12. The pulser 3 drives the transducer 4 in synchronization with the fall of the rate pulse b. When the transducer 4 is driven, the transducer 4 transmits an ultrasonic wave into a living body 5. The ultrasonic wave propagates in the living body 5 and is reflected on a vascular wall 6 or blood corpuscles or cells (in FIG. 1, only the blood corpulscle designated) by reference numeral 7 is indicated by a thick black point, and other blood corpuscles are indicated by small points). The echoes d are received by the transducer 4, which converts the echoes into an electric signal of the magnitude corresponding to the intensity of the echoes. The converted echo signals are inputted to a preamplifier AMP 9, and are amplified to the suitable amplitude. The amplified echo signals are then inputted to a mixer MIX 10. To the MIX 10 is inputted a reference signal of the frequency corresponding to the central frequency of the ultrasonic wave transmitted from the transducer 4, from the clock pulse generator 1. The echo signals are mixed by th MIX 10 with the reference signal from the generator 1. The mixed signal is in turn inputted to a low pass filter LPF 11, which removes the harmonic component of the mixed signal. The echo signals thus fed through the low pass filter LPF 11 are in turn inputted to a sample & hold (S/H) circuit 13, which samples only the echo signals from the position to be measured in accordance with the range gate pulse from the range gate circuit 12 as a sampling signal. The echo signals sampled are held at the S/H circuit 13 until the S/H circuit 13 receives the next range gate pulse. The echo signals sampled are in turn inputted to a band pass filter BPF 14, which removes the harmonic wave components produced by sampling, echo from a stationary reflector such as a vascular wall, and Doppler frequency shift signals from a moving article moving relatively slowly to thus sample only the Doppler frequency shift signals due to the blood flow. The echo signals from the band pass filter BPF 14 are then inputted to a frequency analyzer 15, which is composed, for example, of an FFT (fast fourier transformer), and which frequency-analyzes the echo signals to produce a frequency spectrum corresponding to a blood flow signal. The frequency spectrum from the band pass filter BPE 14 is in turn inputted to a monitor 16, which then indicates as an intensity a blood flow signal.
Blood flow information is obtained by the conventional blood flow meter shown in FIG. 1 by the above-described operation.
Since the conventional ultrasonic pulse Doppler blood flow meter however produces the blood flow information in the format of frequency (Doppler frequency shift) as described above, its circuit arrangement is complicated, and checks for the operation of the flow meter is accordingly complicated. As a conventional operation checking device, there is known "The Doppler Signal Simulator for Ultrasonic Pulsed Doppler System" described on Japan Ultrasonic Medical Society, Bulletin, 38-C-24 issued in April, 1981. This simulator employs as an echo signal obtained from a moving article an electric sinusoidal burst signal and obtains a Doppler frequency shift by varying the phase of the burst signal.
Since the burst signal thus obtained is not however an actual echo, this device cannot generally check together with the characteristics of the ultrasound field and the transmitting & receiving circuits according to a transducer.